At what point is it no longer okay to talk about your students' asses?

I've recently started substitute teaching in the state of New York. A few years ago I worked for a half year as a substitute teacher in my hometown in California, where I have some fond memories (a whole gamut--high school agriculture, special ed P.E., remedial math, ESL, art history) and some not-so-fond memories (junior high school anything, especially woodshop). Because you have to have teacher certification to sub at public schools in NY, I've been going through an agency that deals with private and charter schools. Since there are more charter schools for elementary students, I've been called in primarily to work with the little ones. This week was exclusively kindergarten and first grade. Not my forte. I don't know how to speak to them. I'm getting over it, but after sexual harassment fears, I was really uncomfortable repeating phrases like, "put your bottoms in your chairs"--a necessary mantra, I've learned. We do NOT share the same sense of humor. I'm used to handling kids twelve and up by wielding sarcasm instead of detention. But try using sarcasm on a first grader? S/he will retort with a fart joke and send the classroom into stitches.

However, I do feel lucky to have these privileged glimpses into what must be the essentials of human nature, unmitigated by social pressure.

Little kids are uninhibited. One moment, they're hugging you--the next, they're bawling. They are completely unabashed in their pursuit of attention. Talk about wearing your heart on your sleeve. Six-year-olds, by and large, are incapable of concealing their true emotions. At what point did we all learn how to hide our feelings? Okay, sure, maybe having a temper tantrum gets in the way of some notion of "productivity," but why do we encourage kids to "express themselves" and then punish them for doing just that, whenever their expressions are excessive?